Advent is the period of time starting 4 Sundays before Christmas. Sometimes it starts in November, other times it starts in December.
Christians will often do extra Bible reading during this time. Churches may have special readings at this time.
There is a custom involving an Advent wreath in which candles are lit, starting with one candle the first week, and progressing with additional candles each week.

The Jesse tree is based on Isaiah 11:1 and is a reminder of God's work through the Old Testament and His promise of a Saviour. Typically symbols are adding each day or week through the month of Advent.